I've fixed this for the next version by resampling files to use sample rates with known filter coefficients. It should work for files at any sample rate and will try to use integer divisor sample rates (e.g. for 88.2 use 44.1 or for 192.0 use 48.0).

That is expected since the files are resampled. The best other source for replay gain that I'm aware of is FLAC, and for sample rates above 48.0 the new FLAC will also resample, but by throwing away samples (no interpolation or similar). Since the replay gain is only an estimation of the perceived loudness it should be OK.

It's not as practical for the new Tag to display all the embedded tags like it was for the old one because of the various file formats supported. The names of ID3v2 aren't human-friendly (e.g. TALB), mp4 uses atoms with four character names, and FLAC/Vorbis/APE have more user-friendly formats.

Also, I don't understand your request to move the Description field from the Artwork tab to the Misc tab. In the Artwork tab Description is a place to enter a short amount of text describing the image, and it doesn't make sense to select an image in the Artwork tab and then be forced to move to the Misc tab to edit the image's description.

re "Description": I have never used it myself, but in files I buy (from Qobuz, usually) doesn't seem to be used for anything related to Album art: looks more like a duplicate/extension of the Comment tag. that's why I thought it was better suited in a different "group"